Most "real" acrylic fabricators buy standard acrylic sheets by the pallet. UV-filtering acrylic is a product most acrylic fabricators buy in small quantities, and their cost is therefore disproportionately higher.
Check with the acrylic fabricators who cater to framers, such as Gemini/Showcase Acrylics in Chicago and Superior Acrylic Framing in California. These folks may have a better deal on the UV-filtering boxes.
I'm not sure about Superior, but Gemini/Showcase can now build acrylic boxes out of Optium Museum acrylic, which has anti-reflection coatings as well as UV-filtering...almost invisible.
Another alternative would be to build a glass box, using the specially-shaped, wooden edge mouldings made a supplier such as Southwinds.